The present invention relates to a variable cycle propulsion system for a supersonic airplane making it possible firstly to obtain high thrust with a high bypass ratio during takeoff, landing, and subsonic cruising flight in order to greatly reduce noise during those stages of flight and to improve specific fuel consumption, and secondly to obtain a high exhaust speed adapted to supersonic cruising flight.
More particularly, the invention relates to a propulsion system that has two different configurations: one configuration for takeoff, landing, and subsonic cruising flight; and another configuration for supersonic cruising flight.
When designing a supersonic commercial airplane, there arises the particular problem of low airplane engine noise during takeoff, climbing, and landing. In order to be certified, all airplanes must nowadays comply with low noise regulations for takeoff and landing.
Furthermore, supersonic airplane engines must also satisfy requirements of low engine pod drag during supersonic cruising, low specific fuel consumption while overflying inhabited areas at subsonic cruising speed, and reduced emissions of nitrogen oxide pollution close to the ozone layer at high altitude.
In order to satisfy these various requirements, engine manufacturers have proposed a variable cycle engine for propelling supersonic airplanes. Typically, that type of engine comprises a gas generator and at least one fan, the fan enabling the bypass ratio of the engine to be adjusted and thus enabling noise to be reduced. The engine adopts two different configurations: a configuration for subsonic cruising flight, takeoff, and landing using high bypass ratio; and another configuration for supersonic cruising flight with a low bypass ratio since there exists a degree of incompatibility between those two stages of flight in terms of engine operation.
The requirement for low engine noise during takeoff and landing implies in particular a low gas exhaust speed during takeoff and at subsonic cruising speed, and that is in contradiction with supersonic cruising flight which requires gas to be exhausted at high speed.
Noise level depends on gas exhaust speed, and to reduce noise to an acceptable level, exhaust speed must nowadays be less than 400 meters per second (m/s), which corresponds to a threshold of 103 decibels (dB) (with new regulations reducing this to 300 m/s or 90 dB as from the year 2006). Such an exhaust speed thus implies an engine having low specific thrust, which corresponds to a large bypass ratio, i.e. to a high level of drag when cruising at supersonic speed.
Thus, the variable cycle engines proposed by manufacturers seek to combine low engine noise during takeoff and landing, low specific fuel consumption during subsonic cruising, and high specific thrust during high altitude supersonic cruising.
Various variable cycle engine designs are known, however varying the bypass ratio of such designs does not enable optimization to be good, both in the subsonic configuration and in the supersonic configuration.
Adopting a gas exhaust speed that is equal to or less than 400 m/s requires an engine pod to be of large diameter, and all presently-known variable cycle engines, and in particular, when the fan is connected to and integrated in the engine, those described in French patents Nos. 2 513 697, 2 688 271, and 2 685 385 require the pod to present a front section that is larger than that which is optimum for supersonic cruising flight.
For example, U.S. Pat. No. 5,529,263 discloses a supersonic airplane having a propulsion assembly for takeoff, landing, and subsonic cruising flight, and two engines adapted for supersonic cruising flight. The propulsion assembly is constituted by retractable high-bypass ratio booster turbojets, and that presents numerous drawbacks, in particular concerning bulk and weight for the airplane.